Bridge mode does not do what is explained

I had similar problems with xfi and Nighthawk, I WAS able to resolve.

⛑ HOUSEKEEPING BEFORE I RECOMMEND ANYTHING, have a roll back plan before you start & enlist help or do more homework if you aren't sure you know how to go back to original setup that was working, poorly but working. If you are experienced, ignore the repetition but since others likely don't have your experience you understand why I'm harping on backup and recovery strategies.

⚠ IMPORTANT BEFORE CHANGING ANYTHING! download a backup of your modem and routers configurations TO ROLL BACK IF NEEDED *

⚠ BIG CAUTION ⚠
⚠ BIG CAUTION ⚠
If you use xfinity security, phone, cameras or other services you may NOT BE ABLE to successfully use modem in bridge mode because xfinity TV boxes, phone & security do use internet SETTINGS & XFINITY MODEM FEATURES contrary to popular belief.

🛑 DO NOT TEST CALL 911 EVER EVER EVER! 🛑
But definitely TEST, TEST, TEST & VERIFY OTHER SERVICE (ESPECIALLY PHONE LINE FOR ALARM & LAND LINE FOR 911 Calls are working before & after changes, again BUT DO NOT TEST CALL 911 EVER without an approved test call appointment from 911.org, big fines and jail are frequently imposed for test 911 calls because by law they must send police if you tell them you are just calling to test without proper prior authorization.

AFTER ANY CHANGES make a new backup of ALL CONFIGURATIONS, routers & modems, BOTH before and after changes (I use different name with date & time so I can revert later if problems are found i.e. Xfinity_Modem_cfgBup_04MAY020_2PM_Before)

It should go without saying but put the configurations on a USB drive you can access even if your internet is down & keep copies for awhile in long term storage like cloud drive. But remember cloud does you no good if internet is unreachable.

OK, NOW TO THE FIXES, if you choose not to use above advise I can rest knowing that you were ⚠ warned ⚠

I realize it shouldn't be necessary because bridge mode is supposed to turn off all other services, but remember xfinity leases out its public Wi-Fi, including the "public" broadcasted "xfinity" ssid from any consumer modem where the customer does not specifically forbid or disable sharing. It's how they claim the coverage they claim legally and they also use it as preferred network for xfinity cell phone service which is really a Wi-Fi phone service that falls back on Verizon only if no xfinity Wi-Fi is available.

That said, these steps fixed my dropped internet connection issues on modem, they still don't provide the gigabit speed we pay for but that's for a different post.
1. Disable "xfinity" shared public Wi-Fi
2. Disable private Wi-Fi on xfinity router
(you should now have no wireless signal being advertised / broadcast from xfinity modem, use phone or phone app to scan for Wi-Fi signal)
3. Put xfinity modem in Bridged mode, save and reboot.
(important, verify xfinity did not turn Wi-Fi back on during reboot, occasionally their system updates reset Wi-Fi "especially the public shared Wi-Fi" back to on during reboot.
4. Do not connect personal router until after above reboot step #3 is completed.
Connect your Nighthawk (or other router) and configure as if it is connected to a regular, generic, cable modem.
5. Configure your nighthawk Wi-Fi and other settings as desired, generally the default settings work but obviously change passwords.
6. I've found by replacing the primary xfinity DNS 75.75.75.75 & 75.75.76.76 with a public DNS like Google 8.8.8.8 or yahoo, etc, service response is much better.
I would however keep the 2nd DNS as an xfinity DNS because xfinity TV boxes & security do use internet settings.